The invention relates to a d.c. subfractional horsepower motor with preferably permanent magnet excitation and variable rpm adjustment, particularly for the drive of dental tools.
In d.c. subfractional horsepower motors it is usual to change the rpm via electrical elements. A change of the rpm is known either through the change of the armature voltage or (in motors with stator windings) by changing the magnetic flux with the assistance of resistance elements.
In d.c. subfractional horsepower motors with permanent magnet excitation and normal construction (with cylindrical rotor), an rpm change or control exclusively by means of changing the armature voltage is known. In dental technology, for example, where one employs such motors for the drive of drilling, grinding, or cutting tools, such motors are operated over an rpm range of approximately 4,000 through 40,000 rpm given a rated voltage of approximately 24 volts. A change of the rpm ensues from a foot switch via a changeable resistance.
It is often desirable to increase the extant rpm range either toward an upper speed range or toward a lower speed range. This is not satisfactorily possible solely by changing the armature voltage, because, for example, in the lower rpm range a power output that is too small, or a torque that is too small would be supplied at the motor drive shaft. Up to now, an expanded rpm range without undue power losses, or without excessive torque losses could only be achieved, essentially, with the assistance of a change of mechanical gear ratio. In dental technology, for example, in order to be able to expand the aforementioned rpm range of the motor of approximately 4,000 through 40,000 rpm up to about 120,000 rpm and down to approximately 800 through 1,000 rpm a suitable handpiece with reduction or translation gears is placed on the drive motor.
However, gears in and of themselves and particularly given such small dimensions and relatively high rpms as for example in the said dental handpiece drives represent wearing parts which are particularly in need of maintenance. Moreover, they require a relatively large fabrication outlay; because of the relatively small space available for housing a gear in such subfractional horsepower motors and, particularly, in those for driving dental handpieces, especially high demands are made with respect to material and observation of the fits of the gear parts. A further disadvantage is to be seen in the frequent change of the handpieces which is required when the dentist must work with different rpm ranges during the treatment of a patient.